Interpretive Summary: Lactic acid bacteria (commonly called inoculants) are often added to crops at ensiling to obtain good-quality silage and improve milk production. However, it has been a mystery as to why inoculants cause cows to produce more milk. Recent laboratory research suggests that rumen bacteria (bacteria from the main stomach of the cow) grow better on some inoculated silages. Rumen bacteria help the cow in feed digestion and are a major source of protein for the cow and the milk she produces. In the current study, we investigated how an inoculated silage affected cows and their rumen bacteria. The makeup of the rumen bacterial population was different between the cows fed inoculated and untreated silage. We also found differences in the rumen bacterial makeup between cows that produce low levels of milk fat and those with normal production of milk fat. While this study still has not solved the mystery as to why inoculation of silage increases milk production, it does indicate that changes are taking place in the rumen bacterial population, and these shifts are likely tied to solving the puzzle. If we can understand how inoculants affect the cow, it should lead to the development of more effective silage inoculants and help farmers produce silages that are more efficiently and effectively utilized by their cows.

Technical Abstract:
Some silage inoculants promote an increase in milk production, possibly through altering the rumen microflora. In this study, dairy cows fed alfalfa silage treated with the inoculant, Lactobacillus plantarum MTD/1 (LPS), were compared to cows fed untreated silage (Ctrl) with the objectives: 1) to determine the effect of inoculant on rumen bacterial community composition (BCC), and 2) to quantify the relative population size (RPS) of the inoculant in rumen ingesta. Eight ruminally cannulated Holstein cows were allotted to two diets (Ctrl or LPS) in a double cross-over design with four 28-d periods. Diets were formulated to contain (per kg DM) 280 g NDF and 162 g CP, and contained (g/kg DM): alfalfa silage, 509; corn silage, 206; high-moisture shelled corn, 214; soy hulls, 47; plus minerals and vitamins. Ruminal digesta were collected just before feeding on the last 3 days of each period and were separated into solid and liquid phases. Microbial DNA was extracted from each phase, amplified by polymerase chain reaction (PCR) using domain-level bacterial primers, and subjected to automated ribosomal intergenic spacer analysis (ARISA) for comparison of BCC. Total VFA (mM) tended to be greater for LPS compared to Ctrl (P = 0.08). Individual VFA were not influenced by the diets, but showed a significant cow effect. Milk fat content and yield showed a significant cow effect (P < 0.01), but were not influenced by the diets. Ruminal acetate (mol/100 mol) and acetate:propionate were lower and propionate (mol/100 mol) greater for the 2 milk fat-depressed (MFD) cows compared to the other 6 cows. Correspondence analysis of the 265 peaks in the ARISA profile across the 188 samples revealed that the first two components contributed 7.1% and 3.8% to the total variation in the profile. The ordination points representing the liquid and solid phases clustered separately, indicating that these phases differed in BCC. The LAB counts (log10 cfu/g of wet silage) were 3.94 and 4.53 for the untreated and treated silage, respectively. The RPS of L. plantarum, determined by qRT-PCR of 16S rRNA gene copies, was greater in LPS compared to Ctrl (P < 0.01). The ordination points corresponding to certain individual cows clustered separately, and the most distinctive bacterial communities were those associated with MFD cows. The RPS of the bacterial species, Megasphaera elsdenii, was greater in MFD cows, although mean RPS of M. elsdenii did not differ between the treatments. The results indicated that silage inoculants containing L. plantarum can affect rumen BCC beyond elevating the population of the specific microbial inoculant.